A universal joint, in general, is a mechanical coupling between two rotating shafts, used to transmit power, motion, or both. The axes of the shafts are always intersecting and the bending angle between them is permitted to change during operation. The "Cardan joint" (also called "Hooke's joint") is a well known example of a universal joint. It is noted that the Cardan joint transmits rotary motion but does not ensure that the angular velocities of the shafts are equal at all times.
A constant velocity universal joint, on the other hand, is a universal joint that transmits a rotary motion while keeping the angular velocities of the shafts equal at all times.
True constancy of angular velocity transmission is achieved by a widely used arrangement of two Cardan joints in series, the output member of the first joint comprising the input member to the second joint. It is noted, however, that the constancy is kept only under strict geometrical requirements: both input and output shafts must lie in one plane and both bending angles of the two Cardan joints must be kept equal at all times. This principle is used in the so called Double Cardan Universal Joint, as described for example in U.S. Pat. Nos. 4,57,243 and 5,419,740, where two Cardan joints are connected by a relatively short intermediate member designed to meet said geometrical requirements.
Another known constant velocity universal joint, which is commonly used in motor vehicle front-wheel drives, is the Rzeppa joint. It operates on the basic principle that constancy of transmission is ensured when the contact point or points between the two shafts lie in the so called "homokinetic plane" of the joint. This plane is normal to the plane defined by the two shaft axes and lies along the bisector of the angle between the two shaft axes. The Rzeppa joint consists of a cage that keeps six balls in the homokinetic plane at all times. A further development of the Rzeppajoint, that allows not only angular but also axial relative movement of the shafts, is described in U.S. Pat. No. 4,573,947.
Examples of other types of universal joints, or constant velocity universal joints, are described in U.S. Pat. Nos. 4,786,270, 5,474,500, and 5,256,107, as well as European Patent Publication 668 452 A1.